However by the time the M6 was ready for production, the Armored Corps had lost interest in the project. The advantages the M6 offered over medium tanks - its much thicker armor and slightly more powerful gun - were offset partly by the shortcomings of the design - such as very high silhouette, awkward internal layout and reliability problems - and partly by logistical concerns. By the end of 1942, the Armored Corps were sure that the new M4 Sherman gave adequate solution for the present and the near future, while being reliable, cheap and much easier to transport.

Work on M6 didn't stop at once. The T1E1 prototype was tested with a T7 90 mm gun and was found to be a satisfactory gun platform, although poor turret layout was noted again. In August 1944 the Ordnance recommended using the T1E1s produced to build 15 77-ton vehicles designated M6A2E1, with thicker (up to 7.5 inch vertical protection) glacis armor and a turret developed for the T29 Heavy Tank, armed with a T5E1 105 mm gun. The proposal was rejected by General Eisenhower. However, by late 1942 main development effort shifted to other projects, one of which eventually resulted in the M26 Pershing.

On 14 December 1944 the M6 was declared obsolete. Only forty units were produced and they never left US soil. All were eventually scrapped except for a single T1E1 which is on display at the United States Army Ordnance Museum, Aberdeen, Maryland.